ALMA Sideband Separating Receiver
The Arizona Radio Observatory’s 1.3mm JT receiver system is the first of its kind to incorporate the latest SIS mixer technology; image–separating mixers with internal IF amplifiers (mixer–preamp module). These mixers were developed at the Central Development Laboratory of the National Radio Astronomy Observatory as part of the ALMA project, and the ARO has had the privilege to be the first observatory to integrate these mixers into a radiometer system for astronomical observations. These mixer–preamps have demonstrated state–of the–art performance for sensitivity, and have no moving parts which makes them far easier to use than the conventional receiver systems using quasioptical techniques for image separation and are far more sensitive.
Image separating mixers do not operate in the conventional way quasioptical single sideband receivers do. In the traditional quasioptical SSB system, the mixer is still being used as a DSB mixer where both sidebands are downconverted. The difference being that the image sideband is terminated in the cold dump of the optical system. This image noise is still downconverted thus adding a considerable amount of noise in the signal port. In an image–separating mixer, the image noise and signal is truly separated from the signal noise thus isolating it from the desired sideband. This means that for an ideal image–separating mixer, the noise performance would approach that of a DSB mixer except from the additional 4 K of added noise from the cold termination of the image port of the RF hybrid
This radiometer system is unique over its predecessor because both sidebands are available simultaneously (thus, the term image–separating mixer). This means that for a dual polarization system, four independent IF’s are available. This can be extremely advantageous because it can allow the observation of two separate spectral lines simultaneously, thus reducing the observation time in half. The instantaneous IF bandwidth of each channel is 4 GHz (from 4 to 8 GHz), but capable of providing 8 GHz of IF bandwidth (from 4 to 12 GHz). This amount of IF bandwidth has made the system back end limited because of the limited amount of bandwidth available from the current spectrometer systems at the SMT.
The current realization of the front end is an interim system. It uses the old 1.3mm quasioptical JT Dewar and cross–grid (at room temperature) to separate the two orthogonal linear polarizations. Two inserts are used for each polarization. Improved performance will be achieved if the room temperature cross–grid which is known to add a non-negligible amount of noise. Later, a waveguide OMT will be used which will enhance the sensitivity, and dramatically reduce the size of the front end because both polarizations will be attached to the OMT. This improved package will later be integrated into a new much smaller Dewar. This system is intended as a facility instrument primarily for spectroscopic observations.
Due to the immense amount of work that needs to be done in such a short period of time, implementation of the receiver system will occur in 3 phases. Phase 1 will only have two IF channels available at one time. This means that the user will be restricted to two observing modes: either USB or LSB for each polarization, or both sidebands but with only one polarization. Full IF channel steering and sub-band selection will be available.
Phase 2 will implement full four channel capability. This system will replace the existing two-channel IF processor with a four-channel system. This will allow independent accessibility of each of the four IF outputs from the front-end. A four channel total power facility continuum system with “zero check” capability and external four channel test ports will be used for all of the front ends in both receiver rooms. This will replace the existing two channel continuum system on the left side and the 2 channel IF processor unit in the receiver frame currently being used in phase 1. Full spectrometer configuration will also be available. This will allow the spectrometers to be arranged in a series or parallel mode with respect to each other.
Phase 3 will have the same capability as Phase 2, but will incorporate the left side receiver room into the upgraded IF system. This includes the 345 and 490 GHz receivers. The 345 GHz receiver will change its IF from 1.5 GHz to 4-8 GHz, and the 490 GHz receiver will still use its 1.5 GHz IF. An upconverter will be installed to convert the 1.5 GHz IF center frequency to 6 GHz.
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